1. Technical Field
The present invention relates to a variable wavelength interference filter that selects and outputs light of a desired target wavelength from incident light, an optical module that includes the variable wavelength interference filter, and an optical analyzer that includes the optical module.
2. Related Art
Generally, a variable wavelength interference filter is known in which reflective films are arranged on the opposing faces of two substrates so as that the reflective films face each other across a gap having a predetermined dimension (for example, see JP-A-7-286809).
In the variable wavelength interference filter disclosed in JP-A-7-286809, driving electrodes are arranged on the faces of two substrates so as to face each other, and, by applying driving voltages to the driving electrodes, a gap interval can be adjusted in accordance with an electrostatic attractive force. Here, one substrate includes a low-rigidity portion located on the periphery of the reflective film. The low-rigidity portion is formed to be thinner than that of the other portions of the substrate and the driving electrode is also disposed on this low-rigidity portion. In accordance with the transformation of the low-rigidity portion, the flatness of the reflective film disposed on one substrate is secured. According to such a configuration, the variable wavelength interference filter can transmit light of a predetermined wavelength in accordance with the gap interval.
Since the variable wavelength interference filter transmits light of a desired wavelength by adjusting the gap interval, high gap precision is required.
However, when the driving electrode is formed on the substrate, an internal stress acts on the driving electrode in the facial direction (a direction extending along the substrate surface) of the driving electrode. The magnitude of the internal stress and the direction in which the stress acts are dictated based on the method of forming the driving electrode as a film, the material of the film, and the like. Here, in a case where compressive stress acts on the driving electrode formed on one substrate, the substrate is convexly bent toward the other substrate. On the other hand, in a case where tensile stress is applied to the driving electrode formed on one substrate, the substrate is convexly bent in a direction away from the other substrate. Particularly, since the low rigidity portion of the substrate has lower rigidity than the other portions, in a case where the driving electrode is formed on the low-rigidity portion, there is a concern that the amount of bending of the substrate may be large.
When the substrate is bent in accordance with the internal stress of the above driving electrode, in an initial state in which no driving voltage is applied to the driving electrode, there is a problem in that bending also occurs in the reflective film so as to decrease the resolving power of the variable wavelength interference filter.